The Key to Carburetor Maintenance is to Check the Pilot Screw!

The intake system in current motorcycle sold in newer models is an electronic fuel injection system = fuel injection (FI), but the carburetor was the mainstay of pre-2000 models. It's a smart component that uses a combination of jets and needles to mix air and fuel, but its performance can deteriorate over time. If the pilot screw, which is important for idling and city driving, gets dirty, it can cause problems and problems, so it needs to be addressed properly.

Pilot screw with the low opening angle

The pilot screw adjusts the amount of air mixture, whereas the air screw, which adjusts the same idle area, adjusts the amount of air before it mixes with gasoline. When you want the mixture to be thicker, the pilot screw opens to increase the absolute amount of mixture, whereas the air screw closes to reduce the air and increase the amount of gasoline.

The role of the carburetor is to control the amount of air the engine draws in through the throttle valve, which is opened and closed by throttle operation, and to supply gasoline in proportion to the amount of air drawn by the engine. The carburetor does not use a fuel pump to inject gasoline-like fuel injection, but rather makes clever use of the pressure differential between the amount of air passing through the carburetor body (venturi) and the pressure, or in other words, the power of nature.

During the process from idle to full throttle opening, the passage area of the venturi expands with throttle opening and the part or parts of the venturi that are supplied with gasoline change depending on the amount of air being sucked in. Pilot jets (slow jets) in the low throttle opening region, jet needles (straight diameter, tapered and cut up) in the middle region, and opening 3/In 4 and above, it's the main jet, that's what they call it.

When it comes to carburetor settings and tuning, many users focus on the main system. Certainly, the mains settings are important when racing at maximum power and on the track.

But how often do you open the throttle to full throttle when you're in town, touring, or winding down at the speed limit? On a low-powered moped, if you are accelerating at full throttle to join the flow of motorcycle around you, you may have to open the throttle briefly until you hit the throttle stopper. In the 400cc and 1000cc class, there are only seconds when you can open the throttle fully under acceleration.

In other words, when we develop a motorcycle, I set it up for full throttle, but we rarely use the full-throttle range when actually riding on public roads. On the other hand, when riding on the street, we are far more likely to use the slow range or pilot range.

The throttle valve in the carburetor is almost closed at idle with your hand off the throttle grip. The "almost" is because if you shut off the airflow completely, the engine will stop, and you're letting as much airflow as you can idle.

This small amount of air/gasoline mixture exits through a channel called the pilot outlet. The pilot screw then regulates the amount of air mixture leaving the pilot outlet.

The pilot screw, which has a thin needle-like tip, adjusts the amount of mixture that passes through it depending on the number of revolutions at which it is returned (opened) from its tightened position. The number of revolutions at which it is returned varies slightly from model to model, but it is designed to produce a setting between 1-1/2 and 2-1/2 revolutions.

Gasoline or mixture is fed through small holes in the venturi, such as the pilot outlet or bypass port, to keep the throttle valve from stalling when it is closed. If there is a lot of blowbacks, carbon will stick to the venturi and throttle valve, but the carbon blown into the small holes must also be cleaned up.

What happens to the pilot screw over time?

The carbon buildup on the tip of the pilot screw reduces the amount of air mixture that can pass through the pilot outlet, resulting in a thinner carburetor setting. Increasing the screw's revolutions back may temporarily improve the symptoms, but the only fundamental solution is to remove the carbon.

Not only the pilot screw, but also the jets, jet needles, and other internal parts of the carburetor are strictly measured, so do not rub or poke them. Use a zippered plastic bag for foam type carburetor cleaner to minimize waste.

A carburetor cleaner or combustion chamber cleaner that is resistant to carbon stains will clean off some pretty stubborn dirt. If you find the stain removal to be poor, warm up a plastic bag sprayed with the cleaner to activate the chemicals and improve the stain removal.

The pilot screw, which regulates the amount of air mixture, is mounted on the cylinder side rather than the throttle valve, and after years of use and increased mileage, carbon can stick to it. The carbon that is blown back from the engine side adheres to the tip of the screw when it is wet with gasoline and turns black like the tip of a pencil, which can cause problems with the engine and the CO concentration in the exhaust gas.

The tip of the pilot screw is tapered, which is an important metering device that changes the flow rate of the air mixture depending on the amount inserted into the pilot outlet hole. Therefore, if carbon adheres to the tapered part and the area around the pilot outlet, the passage is narrowed and the amount of air mixed in the idling area is reduced, which can lead to unstable idling and lack of power at launch.

Once the throttle is open and the slow-jet metered mixture is in the realm of being fed, the impact of the pilot outlet is reduced, but it's not pleasant to have a moment of thin carburetor setting in a city with lots of go-and-stops.

Carbon buildup is caused by a thick mixture of air, but it is also said to be a symptom that can occur in engines that are difficult to start without a by-starter (choke). If the by-starter is often used not only when the engine is cold, but also when restarting the engine at warm temperatures, the mixture may be too dense, causing incomplete combustion and contributing to carbon buildup.

Carbon is removed with an engine conditioner.

If the pilot screw is dirty, it's reasonable to assume that the pilot outlet has carbon on it as well. It takes a lot of work to attach and remove the carburetor, but spraying carburetor cleaner into the venturi can have a major cleaning effect.

If the pilot screw tip is dirty, you can easily see it by removing the screw from the carburetor. Although the carburetor itself is removed here for clarity, the carburetor can be left attached to the engine as long as the screw is in a position to be removed while it is installed in the engine. However, it is important to tighten the screw until it stops before removing it, and then figure out the current revolutions per minute.

If there is carbon on the tips of the removed screws, clean them with carburetor cleaner or parts cleaner, which is effective in removing carbon. The fine needle tip is the part that measures the mixture, so avoid scrubbing it with a non-woven cloth or steel wool.

If the carbon buildup on the screw is noticeable, you should also clean the pilot outlet side of the carburetor body.

In this case, spraying carburetor cleaner into the threaded hole where the pilot screw was removed will have a certain effect, but removing the carburetor from the engine and allowing the cleaner to penetrate the inside of the venturi will clean it more effectively.

However, in this case, as well as cleaning the screw itself, you should not insert a wire or other object into the pilot outlet and play around with it. A half-turn of the screw can completely change the power of the engine, and if you damage this delicate part, you will not be able to set it correctly.

When the screw and pilot outlet are cleaned and restored, the spring, washer, and O-ring are set at the base of the screw. Of these, the O-ring is very important to prevent secondary air inhalation, so you should be cautious enough to replace it with a new one if it is deformed or hardened, or even if it appears to be intact.

Then screw in the screw until it stops, and return it to the specified standard return speed. At this point, even if it's obviously different from the pre-disassembly return speed, first start the engine at the service manual and find the point where the engine speed is highest. If the engine cannot be adjusted within the standard rev range, further work may be required, such as overhauling the air cleaner or carburetor itself.

However, in many cases, the condition of the low opening region due to carbon deposits on the pilot screw can be improved by careful cleaning, so riders who continue to drive carbureted motorcycles should check it out.